This invention relates to an apparatus for developing an electrostatic latent image, which has been generally employed in the field of electrophotography, and, more particularly, to a magnetic brush development apparatus for use therein.
Conventionally, as far as a developer to be used for the above-described field is concerned, two component developers such as a mixture constituted by magnetizable carrier particles, for example, iron particles, each having a relatively larger particle-diameter (an approximate particle-diameter of 75 .mu.m) and non-magnetizable particles, or one component developers employing magnetizable toner particles and the like are well known in the art. The respective developers inherently have a number of advantages and, have been put into practical application up to the present. Hence, recently, in order to further improve the above-described developers thereby to provide them with much enhanced characteristics, there has already been carried out considerable research, resulting in providing somewhat different novel developers. More specifically, there have been proposed the following two component developers respectively constituted by two components of magnetizable particles and non-magnetizable particles having approximately the same particle diameters with respect to each other, for instance, in U.S. patent application Ser. No. 863,616, filed Dec. 23, 1977, in which a developer constituted by magnetizable toner and electrically insulating non-magnetizable toner at a predetermined mixing ratio by weight is employed for the developer, and also in U.S. patent application Ser. No. 949,426, filed Oct. 5, 1978, in which there is employed a developer constituted by insulating toner particles and carrier granules having the properties of (1) being magnetic, (2) having a high electro-resistivity (more than 10.sup.12 .OMEGA..multidot.cm), and being 5 to 40 .mu.m in size.
Actually, although the respective, improved two component developers described above are principally capable of being used in a conventional magnetic brush development apparatus without any important modification, some properties of the recently improved two-component developers mentioned above are a little different from those inherently belonging to the conventional developers. Therefore, so far as the recently improved developers are employed for the developing process, the magnetic brush development apparatus itself should be arranged to be especially useful for the improved developers. Due to the practical demand as described above, some of the present inventors have proposed a magnetic brush development apparatus, which has been described in FIG. 1 of U.S. patent application, Ser. No. 16,610, filed Mar. 1, 1979, patent No. 4,235,194, and has the following construction.
Referring now to FIG. 1, there is shown a dry process developing apparatus 5 according to one preferred embodiment of the application mentioned above, which generally includes a housing 10 extending the width of a known photoreceptor 1 in the form of a drum and substantially enclosed except for an opening 10b adjacent to the photosensitive or photoreceptor surface 1a of the photoreceptor 1 whereat the development of electrostatic latent images formed on the photoreceptor surface 1a is effected, an outer cylinder or developing sleeve 6 rotatably provided in the housing 10 adjacent to the photoreceptor surface 1a, a rotary magnet or multipolar magnet member 7 rotatably enclosed in the developing sleeve 6, a developer- or developing material stirring device 20 provided in the housing 10, and a developing material supplying device or toner dispenser C disposed above the developing material stirring device 20 for replenishing toner into the developing apparatus 5 in a known manner.
The developing sleeve 6 of cylindrical configuration made of non-magnetizable electrically conductive material such as aluminum is disposed for rotation counterclockwise at approximately 30 r.p.m. in a position close to the surface 1a of the photoreceptor 1 which is also capable of rotating counterclockwise. The multipolar magnet member 7 of roll-like configuration has magnetic poles N and S sequentially arranged around its outer periphery and alternating as shown and is adapted to rotate at a speed of 2000 r.p.m. in the same direction as the developing sleeve 6. More specifically, on the assumption that the developing sleeve 6 has a diameter of 31 mm, the developer obtains a moving speed of approximately 10 cm/sec. in the clockwise direction through rotation of the multipolar magnet member 7, and also a moving speed of approximately 5 cm/sec. in the counterclockwise direction through rotation of the developing sleeve 6, and is consequently moved along the surface of the developing sleeve 6 at a speed of approximately 5 cm/sec. in the clockwise direction.
Meanwhile, at a lower portion of the developing sleeve 6, scraper members or scraping plate members 17 and 18 made of non-magnetizable resilient material, for example, suitable synthetic resin, thin metallic pieces, etc. are fixed to a support shaft 17a for sliding contact with the peripheral surface of the developing sleeve 6 in directions opposite to and the same as the direction of rotation of the developing sleeve 6.
The developer is successively transported up to a position A whereat it is affected by the moving force arising from rotation of the multipolar magnet member 7, through trough-like members 30 provided around peripheral edges of rotary edges of rotary discs 29a and 29b for the developing material stirring device 20, and from the position A, is moved along the surface of the developing sleeve 6 at the speed of approximately 5 cm/sec. in the clockwise direction, with the amount being restricted by a doctor blade 13a provided above and adjacent to the surface of the developing sleeve 6. After once being collected in a developing material collecting zone Q formed between the developing sleeve 6 and photoreceptor 1, the developer is rubbed against the electrostatic latent image preliminarily formed on the photoreceptor surface 1a for developing the latent image.
In the above case, since the developer is subjected to the moving force and disturbed owing to rotation of the developing sleeve 6 even in the zone P which is comparatively widely spaced from the developing sleeve 6 in the developing material collecting zone Q, hardening or solidification of the developer at the zone P is advantageously prevented, which is mainly attributed to the novel construction according to this prior application so arranged as to avoid compression of the developer between the developing sleeve 6 and photoreceptor 1.
The developing sleeve 6 has applied thereto a bias electrical potential, the prevailing polarity of which is arranged to be the same as that given by the electrostatic latent image, so that the surface of the photoreceptive drum is prevented from being fogged.
Impurities such as dust and dirt and the like mixed in the developing material, or developer itself, etc. solidified in the vicinity of the doctor blade 13a, which are not affected or only slightly affected by the moving force arising from rotation of the multipolar magnet member 7, are moved by the rotation of the developing sleeve 6 in a direction opposite to that of movement of the developer, i.e. counterclockwise, and scraped off the developing sleeve 6 by the scraping plate member 18 directed opposite to the direction of rotation of the developing sleeve 6. On the other hand, the developer remaining after the developing treatment is scraped off the developing sleeve 6 by the scraping plate member 17 directed in the same direction as the direction of rotation of the developing sleeve 6 for being returned into a developing material storage tank 8 of the developing material stirring device 20.
Still referring to FIG. 1, in a position above developing sleeve 6, a roll 105 is rotatably provided, with a slight clearance being maintained between the roll 105 and the surface 1a of the photoreceptor 1 for preventing dust from the developer from entering the interior of the copying machine (not shown). The roll 105 is provided, at opposite ends thereof, with rollers 106 each having a diameter slightly larger than that of the roll 105 and contacting corresponding ends of the photoreceptor drum 1 for simultaneous rotation with the photoreceptor 1 and also for proper positioning of the developing sleeve 6 with respect to the photoreceptor drum 1.
The developing apparatus described above has a number of advantages and, is an excellent apparatus for practical use. However, there are still problems involved in the employment of this improved developer constituted by a mixture of magnetizable particles and non-magnetizable particles in a developing apparatus of the above-described type. There is still not avoided the occurrence of a number of undesirable operational defects such as the occurrence of a cloud of dust from unused developer powder. Accordingly, there are still left a number of difficulties in the use of the recently improved developers of the above-described type in the developing apparatus specifically improved as described in the foregoing. More specifically, due to the unsolved difficulties in use of the developer of the above-described type in the developing apparatus described above, there in still often brought about undesirable behavior of the developer in the apparatus as well as a resultant quality-degradation of the developed image, which is mainly caused by the unused powder existing within the apparatus.
In order to overcome the difficulties involved in the use of the developers of the above-described type in the developing apparatus described above, the present inventors have found that the following points should be further improved in respect to the developing apparatus. As for the developers, two kinds which have already been disclosed in U.S. patent application Ser. Nos. 863,616 and 949,426 as described earlier, were employed for the experimental tests carried out.
With respect to the first point to be improved, according to the developing apparatus described above, owing to the occurrence of counterclockwise transportation of some developer as the developing sleeve 6 is rotated in a reverse direction with respect to that given in the course of the developing process, the developer, which has not been scraped off by the scraping plate member 18, is gradually, forcibly deposited between the surface of the developing sleeve 6 and the scraping plate member 17 and, thereby, there is brought about an undesirable depositing of coagulated developer.
With respect to the second point to be improved, there can be considered the defect which is due to the undesirable, free dispersion of the developer and its resultant powder, as described hereinbelow. When particular attention is directed to the part of the developing apparatus whereat the developer retained on the developing sleeve 6 is scraped off by the scraping plate member 17, the developer is brought into a free dispersed condition by a collision between the developer mentioned above and the scraping plate member 17, whereby a cloud of dust from unused powder thus produced is directly spread out either outside the developing apparatus itself or onto the surface of the photoreceptor surface, thereby resulting in generation of a number of undesirable phenomena. More specifically, a part of the dust from the unused powder is electrostatically adhered onto a right, approximate edge-portion of a casing constituting the developing material storage tank 8 in FIG. 1. Referring now to FIG. 2, there is shown a detailed view particularly showing the above-described electrostatic adhering characteristics of the developer. Although the magnetizable particles, which are negatively charged and retained on the peripheral surface of the developing sleeve 6 by means of magnetic force generated by the multipolar member 7, each have a rather small particle-diameter as well as small mass, respectively, the particles dispersed from the periphery as described above do not gravitationally fall, but rather tend to be electrostatically stuck to the edge-portion of the casing 8 due to an electrical potential gradient existing between the developing sleeve 6 and the casing 8 described above. The developing sleeve is maintained in an electrically negative state by impressing a bias electrical potential thereon, whereas the casing 8 is generally maintained at an electrical potential level of zero. Even if the casing or the developing material storage tank 8 is made of electrically-insulating materials such as synthetic resins and electrically-conductive materials such as aluminum, brass etc., occurrences of electrostatic adhering of the magnetizable particles can hardly be avoided. However, it has already been confirmed that the amount of the magnetizable particles adhering is much larger, when the casing is made of electrically-conductive material. Due to its relatively small particle-diameter and mass, each particle (the magnetizable particle) adhering to the right edge-portion of the casing 8 will not roll down along the inner surface of the casing, but rather large amount of the particles adhering to the surface described above are rolled up along the surface by the magnetic force caused by the multipolar magnet member 7, which is specifically indicated by an arrow in FIG. 2. Therefore, unless appropriate precautions are taken, an undesirable situation occurs in which the particles thus rolling up are discharged from the casing through a clearance existing between the casing 8 and the photoreceptor surface 1a. To overcome the undesirable situation as described above, as long as the edge-portion is made of electrically-conductive material and is further impressed with the biasing electrical potential thereby to cause it to be given an electrical potential the same level and polarity as that impressed on the developing sleeve 6, it may be possible to eliminate the electrical potential gradient prevailing between the developing sleeve 6 and the edge-portion of the casing 8. Such being the case, although the absolute amount of developer adhering to the edge-portion is naturally reduced, the complete elimination of the particle-adhesion can not be accomplished, whereby an undesirable dispersing of the developer through the clearance formed between the edge-portion of the casing 8 and the photoreceptive drum 1 is not still prevented. Furthermore, subject to the impression of electrical potential on the edge-portion of the casing 8, the magnetizable particles are electrostatically stretched back from the edge-portion of the casing 8 and thus, the magnetizable particles are in turn spread out in the space existing between the developing sleeve 6 and the edge-portion of the casing 8. Accordingly, the undesirable condition, in which the dust from unused particles thus dispersed adheres to the non-image bearing portion on the photoreceptor, will take place. Thus, as is clear from the description, the impression of the biasing electrical potential is not desirable. That is to say, as a result of the impression of the biasing electrical potential, the magnetizable particles each having a negative polarity will not adhere to the developing sleeve 6 and the edge-portion both having the same negative polarity, but will, on the contrary, adhere to the background portion of the latent image on the photoreceptor surface 1a, which has an electrical potential having the same polarity and at a much lower level in comparison with that of the biasing electrical potential described above. A further defect caused by the dust of the unused powder in the developing apparatus in FIG. 1 is the problem involved in transportation of the developer from the trough-like member 30 to the developing sleeve 6, and the following process wherein the photoreceptor surface 1a is rubbed by the developer thus transported, since they involve further possibilities of causing degradation of the quality of the copies image for a reason which will be described hereinbelow. As schematically shown in FIG. 3, the cloud of dust from the unused powder dispersed in the space enclosed by both upper shielding member C and the peripheral surface of the developing sleeve 6 will consequently adhere to the inner surface Ca of the shielding member C. Therefore, when the amount of coagulated particles thus adhering is beyond a certain amount, they are coagulated with respect to each other and then, fall and stick to the surface of the photoreceptive drum 1. Since the developer thus stuck to the surface of the photoreceptive drum 1 is not always effectively, electrically charged and, therefore, is not satisfactorily removed from the surface of the photoreceptive drum 1 even with the help of the electromagnetic bristle of developer, these stuck mosses of developer will undesirably appear on the resultant copied paper, resulting in producing relatively large black specks. Furthermore, since the region of the photoreceptive drum 1, which is denoted by (A) in FIG. 3, is exposed to the dust from the unused powder, the dust stuck on the photoreceptive drum 1 will also caused the same defect as that described above, producing a number of small specks on the resultant copied paper, owing to the fact that the dust stuck on the photoreceptor surface within the region described above has not been effectively electrically charged and, thereby can not be effectively removed from the surface of the photoreceptive drum 1 so as to constitute a portion of the substantial developing material accordingly. Moreover, in spite of the special arrangement of the roll 105 according to the developing apparatus shown in FIG. 1, since minute, gradual escape of the dust from the unused powder floating in the space denoted by (B) in FIG. 3 from the developing apparatus into the surrounding subsystems constituting the electrophotographic copying machine can not be completely prevented, resultant contamination of such subsystem is unavoidable.
Lastly, in order to completely eliminate the contamination of the copied image, there should be further considered an appropriate control of the developing apparatus in connection with respective functionings of the subsystems constituting the electrophotographic copying machine, so that occurrences of such defects as previously pointed out are substantially prevented. Conventionally, in a developing apparatus of a power image transferring type which is arranged to employ a two component developer, the developing apparatus is conventionally so arranged that either the developing sleeve 6 or the multipolar magnet member 7 is rotated and the developing apparatus itself is arranged to be impressed with a biasing electrical potential having a level at least corresponding to that of the background portion of the latent image on the photoreceptor surface. With respect to the impression of the biasing electrical potential, the actuation to effect the ON-mode of operation is conventionally accomplished either at the instant or a little prior to the arrival of the leading portion of the latent image at the developing station, while the actuation to effect the OFF-mode is conventionally accomplished either at the instant when the trailing portion of the latent image passes through the developing station or a little later on, so that the resultant, copied paper will not be fogged. Furthermore, in order to form the exact latent image on the photoreceptor surface, the relatively longer peripheral surface-portion of the photoreceptor surface other than the portion really necessary for the image light-exposure is arranged to be electrically charged in advance by the corona discharger. Therefore, as a result, there are respective, leading and trailing unnecessarily charged peripheral surface-portions on the photoreceptor surface, with the substantial portion on which the latent image is to be formed being interposed therebetween. Such being the case, if the developing sleeve 6 and/or the multipolar magnet 7 is kept rotating even after the completion of developing process of the substantial portion to be developed, the trailing portion is undesirably developed, whereby not only is there an unnecessary consumption of developer, but also excessive overloads on the respective developer-cleaning members cannot be avoided. Consequently, to prevent the undesirable situations described above, either the rotation of the developing sleeve or the rotation of the multipolar magnet member is arranged to be stopped immediately after the end portion of the latent image is passed through the developing station.
In the course of one step of developing the present invention, the conventional developing control described above was applied as the control for the developing apparatus of the magnetic brush development type using the above-mentioned developer constituted by the non-magnetizable particles and magnetizable particles, with only the multipolar magnet member 7 being rotated. As a result, not only did the respective copied images begin to have extremely degraded qualities in accordance with the increase the number of continuous copying runs, but also there were brought about conditions in which the trailing, unnecessarily charged portions following the substantial latent image portion were developed. This is mainly due to the fact that, since the developers constituted by non-magnetizable particles and magnetizable particles, which have recently been improved as previously described, are provided with relatively higher coherent and adherent natures in comparison with the conventional two component developers, and they are easily coagulated and then solidified on the peripheral surface of the developing sleeve 7 when affected by the environmental damp conditions and/or respective mutual, electrostatic inducing forces arising among the respective particles, and thereby, they are prevented from being effectively transported on the peripheral surface. Consequently, in a developing apparatus wherein the developer constituted by the non-magnetizable particles and magnetizable particles is employed, such simple arrangements for rotating either the developing sleeve or the multipolar magnet member as described above can not effectively bring about the appropriate developing process and, some substantial precautions to prevent the solidification of the developer should be taken for a proper developing process. In addition to the precautions described above, since the non-charged portion on the photoreceptive surface unavoidably has magnetizable particles charged with the same polarity as that of the biasing electric potential adhered thereto due to the biasing electrical potential which is being impressed on the developing sleeve by the biasing voltage source, it is necessary to substantially improve control of the subsystems constituting the developing apparatus, with improvements concerning the actuational mode of the impression of biasing electric potential by the biasing electrical potential source also being included.